Category: Practical Exam

Assessment of planning in practical exam - Part I

5/26/2019

What does Assessment of Planning mean in Practical Exam?

The practical assessment incorporates assessment of Planning which has a weighting of 15% out of the entire paper. One, or more, of the questions may be set and require students to apply and integrate knowledge and understanding from different sections of the syllabus. Many students find it IMPOSSIBLE to do well in this section. And this is not true.

What are the requirements for Assessment of Planning?

Students are expected to be able to:
• identify key variables for a given question/problem
• outline an experimental procedure to investigate the question/problem
• describe how the data should be used in order to reach a conclusion
• identify the risks of the experiment and state precautions that should be taken to keep risks to a
minimum

Any example? It's quite confusing..

A typical question could be:

"Plan and produce a procedure to investigate the percentage yield of copper(II) oxide using appropriate reagents and chemicals in the laboratory. You are given a sample of copper(II) carbonate, P."

Wow! How do we go about doing the question?

It is important to plan BEFORE writing your answers down. Just like planning for a composition. A systematic thinking process would be:

Step 1: Ask yourself this question -> What is the objective of the experiment?
- To investigate the percentage yield of copper(II) oxide

Step 2: THINK of the general approach that you would want to use for your experiment by applying scientific concepts that you have learnt.

Copper(II) carbonate can be decomposed upon heating to form copper(II) oxide and carbon dioxide gas.
A fixed mass of copper(II) carbonate is heated strongly until all has completely decomposed to form copper(II) oxide.
Copper(II) oxide is then left to cool and weighed.
The mass of the cooled copper(II) oxide is then measured to determine the percentage yield of copper(II) oxide.

Step 3: Identify variable(s) and conditions that will affect the experiment.

Variable to be kept constant: Mass of copper(II) carbonate, P

Conditions:

Heating of copper(II) carbonate till constant mass
Size of copper(II) carbonate particles
Clean and dry apparatus

Step 4: Write your suggested procedure as a series of step-by-step instructions.

State the appropriate apparatus to be used.
Describe how you would collect the data.
Describe how the data collected should be processed to fulfill the purpose of the investigation.

In my next post, I will be sharing more on HOW the procedure should be written.

Qualitative analysis - cation test demystified

5/25/2019

How do the precipitates look like?

A picture tells a thousand words.
Tip: My students will save these 2 tables in their phones for reference.

For white precipitates

For coloured precipitates

What should we record for cation test observations?

If a precipitate is formed, record its colour. -> A white precipitate is obtained.
If there seems to be no reaction, record as -> No visible reaction (DO NOT write "No reaction")
When you are asked to add more reagent until in excess, record what happens to the precipitate.

The white precipitate dissolves to form a colourless solution.
The white precipitate is insoluble in excess reagent.

How do we memorise everything?

Although a simplified QA table is given during practical exam, I will always make my students understand this table thoroughly before sitting for the practical exam. If a student requires the QA table during the exam, it only means that he is not ready or confident enough.

Tips: Remember the colours for coloured precipitates first. Followed by white precipitates. Then those with no precipitates.

Many students memorise the table above but do not understand the concept behind. Have you ever wondered why does Ca2+ ions form a precipitate with aqueous sodium hydroxide but not with aqueous ammonia?

Calcium hydroxide is slightly soluble in water and would require a high concentration of hydroxide ions to react with it to form a precipitate. Hence, since sodium hydroxide is a strong alkali, the calcium ions react with the high concentration of hydroxide ions to form a precipitate. Aqueous ammonia being a weak alkali do not have sufficient hydroxide ions and thus, no precipitate is seen.

Why does the white precipitate dissolve in excess aqueous sodium hydroxide to form a colourless solution for zinc ion, aluminium ion and lead(II) ion then?

The white precipitate is the hydroxide of zinc, aluminium and lead(II) which can react with aqueous sodium hydroxide to form soluble complex ions as they are amphoteric hydroxides.

Do put in extra effort for Qualitative Analysis. There is a chance that it may appear both in theory and practical exam!

3 common Sources of error in practical exams

5/24/2019

Many students are uncertain of what sources of error are and what to write during practical exam. This article will give your child more confidence when he has some examples of sources of error in his mind when he sits for the practical exam.

What are sources of error?

Sources of error are errors that are inherent in an experiment due to the DESIGN of the experiment. We need to suggest IMPROVEMENT to reduce the sources of error in the experiment.

What are NOT sources of error?

Sources of error are not due to the mistakes made by the student conducting the experiment.

Today, I am going to share with you 3 aspects of design which can cause sources of error to occur.

1. Apparatus

There are times when an apparatus is chosen due to the ease of use. Hence, the degree of accuracy may be compromised and this can be a source of error for the experiment.

Example:
When a measuring cylinder is used to measure 25 cm^3 of a solution instead of a pipette, we can state the source of error this way -> The measuring cylinder used to measure the volume of solution is not as precise as a pipette. It can only measure accurately to 1 cm^3.

The improvement will be -> Use a pipette instead of a measuring cylinder to measure the 25.0 cm^3 volume.

2. Reagents

The stability of the reagent used can be a possible source of error if

it decomposes at room temperature.
it reacts with oxygen in the air.

Example:
Hydrogen peroxide decomposes slowly into water and oxygen at room temperature even when no catalyst is added. The rate of decomposition increases with light and heat. Hence, the source of error will be -> Since hydrogen peroxide decomposes into water and oxygen at room temperature, there will be a loss of reagent before and during the experiment.

The improvement will be -> Store hydrogen peroxide in a dark glass bottle, away from light and heat.

3. Dependent variables

When the dependent variable is temperature change, the source of error can be caused by the inherent inaccuracy when temperature is measured.

Example:
Heat is always being lost to the surrounding when the surrounding is cooler compared to the reacting mixture. The source of error can be written as -> Heat lost to the surrounding is caused by the poor insulation of the reactants, hence the temperature measured will be lower than expected.

The suggested improvement will be -> Use a polystyrene cup with a cover to minimise heat lost to the surrounding.